Solar cells get two electrons for the price of one, efficiency bonus

For the first time, scientists demonstrate generation and extraction of …

Solar power is one of the leading technologies to produce clean, renewable energy, but photovoltaic solar cells remain uneconomical in many areas. The key to improving photovoltaic economics is more efficiently utilizing the energy that the cells receive, as recently demonstrated by solar cells that utilize a combination of light and heat.

Photovoltaic cells work by absorbing photons and using that energy to shift electrons to the conduction band, which creates an electron-hole pair called an exciton. The key to generating electricity in a photovoltaic cell is separating the electron-hole pair and transporting the electron out of the cell to provide an electric current (called a photocurrent). The energy required to promote an electron to the conduction band is called the band gap energy, and it is generally specific to the material in use.

If a photon with more energy than the band gap energy is absorbed, the excess energy is converted into heat, making it unavailable for conversion to electrical energy. For silicon, the most common photovoltaic material, the band gap is 1.1 eV, meaning that energy is wasted over the entire visible spectrum (2-3.1 eV). Rather than waste all this energy, researchers in this week's Science have developed a solar cell that converts each high energy photon into multiple electrons through a process called multiple exciton generation (MEG).

MEG meets nanoparticles

MEG has been well known and studied in bulk semiconductor materials for decades, but the process is tremendously inefficient and only operates at light frequencies higher than those produced by the Sun—in other words, the effect is useless for photovoltaic applications. However, recent studies have shown that multiple excitons can be generated by a single photon, provided they're stabilized in nanoscale semiconductor particles, which confine the electron-hole pairs to very small volumes (a strong Coulombic interaction between excitons prevents decay into phonon modes).

While MEG can be highly efficient in nanoscale semiconductor particles, it has been difficult to efficiently extract the electrons from the nanoparticles—so difficult that the process has never been measured as an actual photocurrent. To solve this problem, the researchers chemically bonded PbS nanoparticles to specially prepared TiO2 surfaces that were exceptionally clean and had large, atomically flat ledges.

The keys to the process appear to be surface preparation of the TiO2 substrate and choosing appropriate semiconductor chemistries so that there is a strong overlap between the conduction bands of the two materials. Electrons will flow provided that there's a slight energy decrease from the semiconductor conduction band to the TiO2 conduction band.

Photoelectric current as function of photon energy was measured on single crystal TiO2 coated with a monolayer of PbS nanoparticles. The experiments conclusively showed that multiple electrons were generated and extracted from the nanoparticles for each high energy photon that was absorbed. The result is a theoretical efficiency increase from 31 percent in single junction photovoltaic cells to 47 percent in a MEG cell.

Because the photocurrent measurements were taken on monolayers of nanoparticles, most of the incident light was not absorbed. In order to make a truly efficient cell that absorbs most of the incident light, they'll have to demonstrate that MEG works through multilayer ensambles of nanoparticles. Efficient MEG and conduction has been demonstrated in more complex nanoparticle structures, but current has not been successfully extracted.

An exceptional paper, in a good way

The technology demonstrated in this paper is particularly interesting for several reasons. First, it is a true “nanomaterial” application where the size of the semiconductor particles enable truly unique properties by confining the excitons to quantum length scales. During my daily abstract scan, it is all too common to find "nano-" papers that simply involve small particles rather than truly novel properties enabled by the scale of the materials.

The work also concentrated on extracting electrons from the nanoparticles rather than just trying to break efficiency records for electron generation. There are constant reports of efficiency numbers for photovoltaic materials that are generated without separating excitons to produce a useable photocurrent. For many types of photovoltaic cells, charge separation is a far more challenging and important scientific problem than charge generation.

Finally, the experimental setup for this study is largely consistent with dye sensitized solar cells, which are easy to manufacture compared to silicon technologies. In principle, this would suggest that the technology used in the study could be rapidly transferred to more industrial scales. I’m skeptical that easy technology transfer is possible, though, due to the emphasis on the surface treatment and surface morphology of the TiO2 conductors. It may not be possible to develop similar surface morphologies on the titania particles and porous structures common to commercial dye sensitized cells.

Despite the potential problems listed above, this study is interesting because it clearly demonstrates an incremental but fundamental step forward for one the most exciting next-generation technologies in photovoltaic devices.

Does the paper say anything about where their funding comes from? (I don't have time to read it today) I'm curious if this discovery has any relation to the additional funds allocated to green research. If so, I would happily say it looks like it's worth it so far.

It pretty exciting to be living in the times when we will figure out how to stop drilling into the earth for energy. We'll be able to amaze our grandchildren with stories of "fossile fules" and "global climate change."

For silicon, the most common photovoltaic material, the band gap is 1.1 eV, meaning that energy is wasted over the entire visible spectrum

Wait, really? Silicon photovoltaics don't generate any electricity at all from visible light? I knew that different materials had different absorption bands and thus peak efficiency at different wavelengths, but this is news to me.

We're starting to get close to that theoretical limit (solar cell efficiencies are about 25% now) for mono-junction. Figuring out how to get that limit higher (along with any increased power per sq meter gains) is the future of solar power.

Does the paper say anything about where their funding comes from? (I don't have time to read it today) I'm curious if this discovery has any relation to the additional funds allocated to green research. If so, I would happily say it looks like it's worth it so far.

It pretty exciting to be living in the times when we will figure out how to stop drilling into the earth for energy. We'll be able to amaze our grandchildren with stories of "fossile fules" and "global climate change."

The authors cite 2 different funding sources, US Department of Energy Basic Energy Sciences and The Department of Energy. Grant numbers are DE-FG03-96ER14625 andDE-FG36-08GO18025 if you want to look up the abstracts for the awards (all federal grants have abstracts attached). It doesn't look like the funds came through green energy programs directly, but the precise flow of money through the DOE can be hard to track. I can tell you that the National Renewable Energy Lab (NREL) in Colorado has been pursuing this tech for some time and has been championing it.

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Quote:For silicon, the most common photovoltaic material, the band gap is 1.1 eV, meaning that energy is wasted over the entire visible spectrum

Wait, really? Silicon photovoltaics don't generate any electricity at all from visible light? I knew that different materials had different absorption bands and thus peak efficiency at different wavelengths, but this is news to me.

Yeah, like Yoweigh said, I mean that all the energy over 1.1 eV is wasted as heat. Si solar cells certainly make use of visible wavelengths. It would have made more sense, from a scientific perspective, to talk about the band gap energy as it relates to solar output, but it didn't seem necessary for this article. I chose to mention visible light because it would be very familiar to everyone and it represents the general peak of the solar spectrum that reaches earth's surface.

Gwenkhan,In a silicon solar cell, each photon collected creates a bit less electrical energy than the silicon band gap (1.1 eV). The solar spectrum is very wide and contains a great deal of energy both above 1.1 eV (including visible) and below 1.1 eV. For photon (i.e. light) energies above 1.1 eV, the EXCESS energy of the photon is not converted to electricity. So when a 3 eV photon is absorbed by the solar cell, less than 1.1 eV of the energy is converted to electricity - the rest is lost as heat.

BTW, Adam was a bit misleading when he stated that "energy is wasted over the entire visible spectrum" - the VISIBLE spectrum isn't the key spectrum: its the entire SOLAR spectrum (i.e. the light coming to the Earth's surface, not just the light that we can see with our eyes) that matters.

In a solar cell made with a single material, there is a balance between collecting lots of photons (i.e. use a material with a low bandgap) and efficiently harvesting high energy photons (i.e., use a material with a large bandgap). For a perfect solar cell made from a single material, the ideal bandgap is a higher than 1.1 eV (its close to the bandgap of GaAs). However, the ultimate efficiency potential for Si is not that much lower than for GaAs. The magic of GaAs is that it is easy to incorporate other materials and make solar cells with multiple bandgaps that efficiently collect the different parts of the spectrum.

Finally, I'll just point out that while the Science article is a great advance, these kinds of technologies are still many years away. In the near (<5-10 years), the main advances are likely to be making existing PV technologies much less expensive while maintaining current efficiency levels.

I think the argument for photovoltaics isn't how efficient they can be, but more that once in place, they can just go and go and go, with minimal maintenance. Last I checked, the sun's going to be around for a very long time. We could be sucking up all that energy hitting the earth. There's a lot of houses with roofs that could be generating their own energy, and being part of an energy grid instead of just leaches sucking off the power plant. Cost is still a huge issue, though. Would be awesome when they finally have a 1 megawatt solar cell for $100 at Wal-mart. Of course, it'll be made in China, so it'll break in 3 months, but I digress.

For photon (i.e. light) energies above 1.1 eV, the EXCESS energy of the photon is not converted to electricity. So when a 3 eV photon is absorbed by the solar cell, less than 1.1 eV of the energy is converted to electricity - the rest is lost as heat.

No. The lowest bandgap of Si is 1.1eV. The silicon indirect bandgap is ~2.5eV which corresponds to ~500nm (the edge of blue/green). The peak of the solar spectrum happens to be at ~500-520nm - matches very well with the indirect bandgap of Si. A very happy coincidence.

So when a 3eV photon is abosorbed, more than 1.1eV worth is extracted. In fact, the peak efficiency of a typical Si solar cell is at 500-600nm. The common claim that everything above 1.1eV is lost as heat is a layman's simplification/fallacy.

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the VISIBLE spectrum isn't the key spectrum: its the entire SOLAR spectrum (i.e. the light coming to the Earth's surface, not just the light that we can see with our eyes) that matters.

No. The vast majority of the energy in the solar spectrum lies in the visible spectrum. The total energy in wavelengths beyond the visible range is, in comparison, negligible. Look up the AM1.5 spectrum and see for yourself. It isn't a coincidence that we have eyes that can see in the energies that are most abundant in sunlight.

Solar power is all well and nice but it isn't going to save the planet because there simply isn't enough land area to server our needs no matter what the PV technology used. Now don't take that as a negative as advances like these can have a very positive impact on peak demand usage, in the areas where solar is actually usable.

This is a big issue, I haven't seen the sun around where I live in almost a week. Solar isn't something you can rely upon locally.

So while this is certainly a good short term strategy it isn't a replacement for research into nuclear and other advanced technologies. The unfortunate thing with respect to nuclear tech is that like climate research it is to tightly controlled by a few people with a narrow view of what nuclear power should be. If there was ever an establishment that needs shaking up, it is the nuclear power research industry.

Grimlog,Without getting into a technical discussion which most Arsians probably don't care too much about, I'd like to respectively disagree.

The lowest bandgap of silicon is absolutely 1.1eV. This is the indirect gap, meaning that silicon absorbs 1.1 eV photons weakly and so it takes a thick piece of silicon to absorb all the light. Silicon does have a direct gap - where light absorption is much stronger - at higher energies. In good silicon solar cells, the quantum efficiency (ratio of photons turned into useful energy to photons in sunlight) is very high from about least 1.2 eV and above. However, Adam S. is correct that all of the energy above 1.1 eV is lost - when a highly energetic photon is absorbed in silicon, it quickly "thermalizes" to the bottom of the conduction band which is 1.1 eV above the valence band. Further losses mean that only 0.75 eV is collected from each photon - even in the very best silicon solar cells.

Nonetheless, silicon is a great material for PV! Its abundant (main component of sand), non-toxic and makes nearly-ideal 1.1 eV bandgap devices.

You are right, of course, that the bulk of the solar spectrum is in the visible. However, there are still lots of photons out there with low energy, in the near-infra-red. And if you want very efficient devices such as those considered in Adam's post, you need to collect those photons as well as the more-abundant ones in the visible.

Grimlog, thank you also for your excellent post on the area required for PV to power our needs.

Even if you manage to cover those relatively small areas with panels, you will still need an electrical storage system with a huge capacity. The sun isn't always shining, so extra power needs to be stored for these times. Bill Gates said in a Ted talk that if we used all of the batteries in the world (car batteries, flashlight batteries, etc), we would only have enough storage capacity power the world for about 15 minutes. We haven't developed the electrical storage technology yet in order to move to a completely solar energy solution. Likewise, we would also need a large, interconnected power infrastructure to distribute the power captured by those widely-spaced solar panel installations. Right now, the world's electrical grids are relatively localized.

Batteries are not a great way to store energy, so the number of them out there probably doesn't matter. If you really wanted to do massive energy storage on a scale like that you'd probably be pumping water uphill, pressurizing air underground, etc.

More likely you'd just use solar during the day, and other sources at night. No one source is going to be able to supply 100% of the worlds energy, so its a little silly to pretend otherwise.

It pretty exciting to be living in the times when we will figure out how to stop drilling into the earth for energy. We'll be able to amaze our grandchildren with stories of "fossile fules" and "global climate change."

Well we could always drill "heat pipes", if we can get deep enough to make use of the thermal energy of the molten rock inside this planet of ours. Then we slap a sterling engine on top and attach a generator. The problem is getting the drill to go far enough...

In terms of solar storage, I can only presume you haven't heard of molten salts? They can act as large batteries, storing power as heat energy overnight and has been in development for some time with a recentdemonstation: http://inhabitat.com/2010/07/22/worlds- ... -at-night/

And the "solar alone" is of course a bit silly. If one extracts out existing renewable sources (hydro, wind) and uses fossil fuels where they make sense (heating your home) then the needed area drops considerably.

I will note that there are numerous problems with the chart, however, like the 2000 kWh/kWp/year figure, which is almost double the average one might expect for those panels in Canada, for instance.

It's also illustrative to use the LLNL numbers of energy use in the US with the amount of land needed to produce that entirely through solar. Its a number that is a WHOLE LOT smaller than the amount of paved road surface in the US, the vast majority of which is less than 50 years old. If the US installed PV at the rate it installed _new_ roads only, it would be solar powered in about 25 years.

Batteries are not a great way to store energy, so the number of them out there probably doesn't matter. If you really wanted to do massive energy storage on a scale like that you'd probably be pumping water uphill, pressurizing air underground, etc.

Maybe. Or maybe NaS batteries, which are being deployed in Japan at a furious rate.

It's also illustrative to use the LLNL numbers of energy use in the US with the amount of land needed to produce that entirely through solar. Its a number that is a WHOLE LOT smaller than the amount of paved road surface in the US, the vast majority of which is less than 50 years old. If the US installed PV at the rate it installed _new_ roads only, it would be solar powered in about 25 years.

On that topic, i encountered a concept whereby roads would operate as solar panels. If that could work, one could even have electric vehicles piggy back of EM from those panels to maintain momentum on long drives, cutting down on the battery needs.

With respect to some of the great work done by solar engineers and scientists - I really think that investing in base load PV generation is a waste of time. You can clearly see that the market has decided against it - as where there is no government support, nobody builds it.

The problem with PV is not that you can't meet our energy needs with it - the problem is that if you took the money needed to do so and invested it in other technologies (non-food biofuels, hydro, geothermal, nuclear, wind) - you'd get massively more utility. So investing in large scale PV is then a lost opportunity.

The one area I think PV shines in - but nobody talks about - is that it's robust. A well build PV cell can operate continously for 60-100 years without repairs or maintainence (assuming no natural disarsters). What other working machine can boast such a long and low maintenance working life?

It pretty exciting to be living in the times when we will figure out how to stop drilling into the earth for energy. We'll be able to amaze our grandchildren with stories of "fossile fules" and "global climate change."

Well we could always drill "heat pipes", if we can get deep enough to make use of the thermal energy of the molten rock inside this planet of ours. Then we slap a sterling engine on top and attach a generator. The problem is getting the drill to go far enough...

Grimlog,Without getting into a technical discussion which most Arsians probably don't care too much about, I'd like to respectively disagree.

In good silicon solar cells, the quantum efficiency (ratio of photons turned into useful energy to photons in sunlight) is very high from about least 1.2 eV and above. However, ... it quickly "thermalizes" to the bottom of the conduction band which is 1.1 eV above the valence band.

I don't believe that thermalization would result in electrons that have undergone direct-band transitions moving to the indirect band edge. Semiconductors can only hold a finite number of electrons at a given energy. In operation, the available slots at 1.1eV will be full or nearly so. This would reduce the likelihood of higher energy electrons from thermalizing to 1.1eV. Many would be collected before that happened.

Also, the shape of the E-K relation in Si means that thermalization would require an energy and a momentum change. Solar cells in operation are hot enough to have plenty of phonons to dirve the momentum change, but the requirement that both changes need to happen simultaneously increases the chance of electrons being swept out of the cell before that happens.

It would be great if this tech could be applied to inexpensive dye sensitized solar cells. Does anyone know if any DSSCs have actually hit the market?

Hi biggeywr8,

You got the right article. The doi and link are now active at the bottom of the article. We always post the DOI, although there is frequently a delay between when the ars article hits and when the doi system registers the article. The link next to the doi at the bottom of the article will explain in more detail.

thank you for a nice summary of a paper i would otherwise have never bothered reading. I studied this a bit a few years back, quantum dot solars cells i think they're also known as, you can sometimes get 3 electrons even but extracting the things was indeed the issue, until now.Another potential benefit of this kind of cell is they don't need rare earth metal compounds to get high efficiency (rare earth metals a ridiculously polluting to extract, and are only mined in china where they don't really care)

With respect to some of the great work done by solar engineers and scientists - I really think that investing in base load PV generation is a waste of time. You can clearly see that the market has decided against it - as where there is no government support, nobody builds it.

The problem with PV is not that you can't meet our energy needs with it - the problem is that if you took the money needed to do so and invested it in other technologies (non-food biofuels, hydro, geothermal, nuclear, wind) - you'd get massively more utility. So investing in large scale PV is then a lost opportunity.

The one area I think PV shines in - but nobody talks about - is that it's robust. A well build PV cell can operate continously for 60-100 years without repairs or maintainence (assuming no natural disarsters). What other working machine can boast such a long and low maintenance working life?

PV will can strive to become a supplementary power source on a large - albeit distributed - scale. It has the potential to supplant peaker plants since its max production is close to peak load time and might also shoulder some of the burden handled by those plants betwene peakers and base load plants (I've always thought of those as "load-following plants", but I'm not quite up on the terminology).

Even without any efficiency miracles seeing production anytime soon, the steady cost declines are making PV solar a real contender for incorporation into building construction - well-situated PV panels can last the life of most structures and generate steadily without the steady influx of operating expenses and significant maintenance a converntional power plant incurs. Inverter replacement every ~10 years is the routine maintenance expense.

Great article. It is an interesting topic, but totally out of my field. I love the balanced technical detail (not too much to drop reading and yet enough to keep me interested) so that I could easily absorb it.

This is nice for space industry but hardly going to be useful on the planet. Problem with solar as with wind is that their source of energy is so spread out and thus you have to spend a lot of effort (pollute a lot) to collect it. That is basic physics that you can not get away from and is reason why solar and wind are doing so badly (except where subsidies are artificially boosting them but that does nothing for the environment).

This is nice for space industry but hardly going to be useful on the planet. Problem with solar as with wind is that their source of energy is so spread out and thus you have to spend a lot of effort (pollute a lot) to collect it. That is basic physics that you can not get away from and is reason why solar and wind are doing so badly (except where subsidies are artificially boosting them but that does nothing for the environment).

I really dont think work(effort) pollution necessarily go hand in hand. At the very least, not a 1:1 ratio. Infact, the reason PV are being considered in the first place is that they have (basically) 0 emmissions!

This is nice for space industry but hardly going to be useful on the planet. Problem with solar as with wind is that their source of energy is so spread out and thus you have to spend a lot of effort (pollute a lot) to collect it. That is basic physics that you can not get away from

I know of no law of physics that requires pollution in order to utilize renewable energy resources.

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and is reason why solar and wind are doing so badly (except where subsidies are artificially boosting them but that does nothing for the environment).

If a portable toilet business is allowed to pour effluent all over your land rather than treating it, it too can put in a lower bid than a business that cleans up after itself, even if the latter business is more efficient.

Hmmm, Lead Sulfide (PbS) nanoparticles. Sounds like an industrial waste disaster waiting to happen if these things ever get deployed widely. Back to the days when we used to inhale lead from the leaded gasoline in our cars.

Hmmm, Lead Sulfide (PbS) nanoparticles. Sounds like an industrial waste disaster waiting to happen if these things ever get deployed widely. Back to the days when we used to inhale lead from the leaded gasoline in our cars.

PbS was chosen for experimental reasons - the process should work with many different, less toxic semiconductor nanoparticles. To make a long story short, the band structure of PbS allowed the researchers to be sure that none of the measured photocurrent was a result of promoting electrons to the conduction band in the TiO2 substrate. The point was to provide concrete, irrefutable evidence of MEG and current extraction, and the choosing PbS eliminated a huge source of experimental error. In real devices, they could ignore this problem.

I will add, though, that many semiconductor materials are quite toxic, so despite the assertions above, your point remains valid.